The enzymes from bioluminescent bacteria contain FMN [4], while the enzyme from Escherichia coli does not [8]. The enzyme often forms a two-component system with monooxygenases such as luciferase. Unlike EC 1.5.1.39, this enzyme does not use NADH as acceptor [1,2]. While FMN is the preferred substrate, the enzyme can also use FAD and riboflavin with lower activity [3,6,8].